1. Quantitative Genetics
Benjamin A. Pierce
GENETICS
A Conceptual Approach
FIFTH EDITION
CHAPTER 24
Quantitative Genetics
© 2014 W. H. Freeman and Company

2. Methods of quantitative genetics coupled with molecular techniques have been used to identify a gene that determines oil content in corn. [Walter Bibikow/Getty Images.]

3. 24.1 Quantitative Characteristics Vary Continuously and Many Are Influenced by Alleles at Multiple Loci
Discontinuous (Qualitative) traits possess only a few phenotypes
Continuous (Quantitative) characteristics vary along a scale of measurement with many overlapping phenotypes
The Relationship Between Genotype and Phenotype
Types of Quantitative Characteristics
Polygenic Inheritance
Kernel Color in Wheat
Determining Gene Number for a Polygenic Characteristic

4. Figure Discontinuous and continuous characteristics differ in the number of phenotypes exhibited.

5. The Relationship Between Genotype and Phenotype
24.1 Quantitative Characteristics Vary Continuously and Many Are Influenced by Alleles at Multiple Loci
The Relationship Between Genotype and Phenotype
Quantitative characteristics
Exhibit complex relationship between genotype and phenotype
May be polygenic
May have environmental influences
Phenotypic ranges may overlap
Cannot use standard methods to analyze

7. Figure 24.2 For a quantitative characteristic, each genotype may produce a range of possible phenotypes. In this hypothetical example, the phenotypes produced by genotypes AA, Aa, and aa overlap.

8. 24.1 Quantitative Characteristics Vary Continuously and Many Are Influenced by Alleles at Multiple Loci
Types of Quantitative Characteristics
Meristic characteristics
Determined by multiple genetic and environmental factors, and can be measured in whole numbers.
Animal litter size.
Threshold characteristics
Measured by presence or absence
Susceptibility to disease

9. Figure 24.3 Threshold characteristics display only two possible phenotypes—the trait is either present or absent—but they are quantitative because the underlying susceptibility to the characteristic varies continuously. When the susceptibility exceeds a threshold value, the characteristic is expressed.

10. 24.1 Quantitative Characteristics Vary Continuously and Many Are Influenced by Alleles at Multiple Loci
Polygenic Inheritance
Refers to quantitative characteristics controlled by cumulative effects of many genes.
Each character still follows Mendel’s rules.
May be influenced by environmental factors.

11. 24.1 Quantitative Characteristics Vary Continuously and Many Are Influenced by Alleles at Multiple Loci
Kernel Color in Wheat
Illustrates multiple genes acting to produce continuous range of phenotypes
Nilsson-Ehle experiment
Intensity of red pigmentation is determined by three unlinked loci
Number of phenotypic classes in F2 increases with the number of loci affecting a character

12. Figure 24.4 Nilsson-Ehle demonstrated that kernel color in wheat is inherited according to Mendelian principles. He crossed two varieties of wheat that differed in pairs of alleles at two loci affecting kernel color. A purple strain (A+A+ B+B+) was crossed with a white strain (A–A– B–B–), and the F1 was intercrossed to produce F2 progeny. The ratio of phenotypes in the F2 can be determined by breaking the dihybrid cross into two simple single-locus crosses and combining the results by using the multiplication rule.

13. Figure 24.5 The results of crossing individuals heterozygous for different numbers of loci affecting a characteristic.

14. 24.1 Quantitative Characteristics Vary Continuously and Many Are Influenced by Alleles at Multiple Loci
Determining Gene Number For a Polygenic Characteristic
(1/4)n = number of individuals in the F2 progeny
that resemble each of the homozygous parents.
n = number of loci with a segregating pair of alleles that affects the characteristic

15. 24.2 Statistical Methods Are Required for Analyzing Quantitative Characteristics
Distribution
Frequency distribution
Normal distribution: a symmetrical (bell-shaped) curve.
Samples and populations
Population: group of interested individuals
Sample: small collection of individuals from the population

16. Figure 24.6 A frequency distribution is a graph that displays the number or proportion of different phenotypes. Phenotypic values are plotted on the horizontal axis, and the numbers (or proportions) of individuals in each class are plotted on the vertical axis.

17. Figure 24.7 Distributions of phenotypes can assume several different shapes.

18. Concept Check 1
A geneticist is interested in whether asthma is caused by a mutation in DS112 gene. The geneticist collects DNA from 120 people with asthma and 100 healthy people and sequenced their DNA. She finds that 35 of the people with asthma have a mutation in the DS112 gene, and none of the healthy people have a mutation in the DS112 gene. What is the population in this study?
the 120 people with asthma
the 100 healthy people
the 35 people with a mutation in their gene
all people with asthma

19. Concept Check 1
A geneticist is interested in whether asthma is caused by a mutation in DS112 gene. The geneticist collects DNA from 120 people with asthma and 100 healthy people and sequenced their DNA. She finds that 35 of the people with asthma have a mutation in the DS112 gene, and none of the healthy people have a mutation in the DS112 gene. What is the population in this study?
the 120 people with asthma
the 100 healthy people
the 35 people with a mutation in their gene
all people with asthma

20. 24.2 Statistical Methods Are Required for Analyzing Quantitative Characteristics
The Mean: the average
The Variation and Standard Deviation
Variance: the variability of a group of measurements
Standard deviation: the square root of the variance

21. Figure 24.8 The mean provides information about the center of a distribution. Both distributions of heights of 10-year-old and 18-year-old boys are normal, but they have different locations along a continuum of height, which makes their means different.

22. Figure 24.9 The variance provides information about the variability of a group of phenotypes. Shown here are three distributions with the same mean but different variances.

23. Figure The proportions of a normal distribution occupied by plus or minus one, two, and three standard deviations from the mean.

24. Concept Check 2
The measurements of a distribution with a higher will be more spread out.
mean
variance
standard deviation
both b and c

25. Concept Check 2
The measurements of a distribution with a higher will be more spread out.
mean
variance
standard deviation
both b and c

26. 24.2 Statistical Methods Are Required for Analyzing Quantitative Characteristics
Correlation: when two characteristics are correlated, a change in one characteristic is likely to be associated with a change in the other.
Correlation coefficient: measures the strength of their association.
Correlation doesn’t imply a cause-and-effect relation. It simply means that a change in a variable is associated with a proportional change in the other variable.

27. Figure 24.11 The correlation coefficient describes the relation between two or more variables.

28. Figure A correlation coefficient can be computed for a single variable measured for pairs of individuals. Here, the numbers of vertebrae in mothers and offspring of the fish Zoarces viviparus are compared.

29. 24.2 Statistical Methods Are Required for Analyzing Quantitative Characteristics
Regression: predicting the value of one variable, if the value of the other is given.
Regression coefficient: represents the slope of the regression line, indicating how much one value changes on average per increase in the value of another variable.

30. Figure A regression line defines the relation between two variables. Illustrated here is a regression of the weights of fathers against the weights of sons. Each father–son pair is represented by a point on the graph: the x value of a point is the father’s weight and the y value of the point is the son’s weight.

31. 24.14 The regression coefficient, b, represents the change in y per unit change in x. Shown here are regression lines with different regression coefficients.

32. Concept Check 3
In Lubbock, Texas, rainfall and temperature exhibit a significant correlation of Which conclusion is correct?
There is usually more rainfall when the temperature is high.
There is usually more rainfall when the temperature is low.
Rainfall is equally likely when the temperature is high or low.

33. Concept Check 3
In Lubbock, Texas, rainfall and temperature exhibit a significant correlation of Which conclusion is correct?
There is usually more rainfall when the temperature is high.
There is usually more rainfall when the temperature is low.
Rainfall is equally likely when the temperature is high or low.

34. Applying Statistics to the Study of a Polygenic Characteristic
Figure Edward East conducted an early statistical study of the inheritance of flower length in tobacco.

35. 24.3 Heritability Is Used to Estimate the Proportion of Variation in a Trait That Is Genetic
Heritability: The proportion of the total phenotypic variation that is due to genetic difference
Phenotypic Variance: Vp
Components of phenotypic variance Vp= VG + VE + VGE
genetic variance: VG
environmental variance: VE
genetic-environmental Interaction VGE
Components of genetic variance: VG = VA + VD + VI
additive genetic variance: VA
dominance genetic variance: VD
genic interaction variance: VI
Summary: Vp = VA + VD + VI + VE + VGE

36. Figure Genetic–environmental interaction variance is obtained when the effect of a gene depends on the specific environment in which it is found. In this example, the genotype affects plant weight, but the environmental conditions determine which genotype produces the heavier plant.

37. 24.3 Heritability Is Used to Estimate the Proportion of Variation in a Trait That Is Genetic
Types of Heritability
Broad-Sense Heritability (H2 = VG/VP)
Narrow-Sense Heritability (h2 = VA/VP)
Calculating Heritability
Heritability by elimination of variance components
(VP – VE = VG)
Heritability by parent-offspring regression
(h2= b or h2= 2b)
Heritability and degrees of relatedness
- H2 = 2(rMZ - rPZ)

38. Figure The narrow-sense heritability, h2, equals the regression coefficient, b, in a regression of the mean phenotype of the offspring against the mean phenotype of the parents.

39. Figure The heritability of shell breadth in snails can be determined by regression of the phenotype of offspring against the mean phenotype of the parents. The regression coefficient, which equals the heritability, is [From L. M. Cook, Evolution 19:86–94, 1965.]

40. Concept Check 4
If the environmental variance (VE) increases and all other variance components remain the same, what will the effect be?
Broad-sense heritability will decrease.
Broad-sense heritability will increase.
Narrow-sense heritability will increase.
Broad-sense heritability will increase, and narrow-sense heritability will increase.

41. Concept Check 4
If the environmental variance (VE) increases and all other variance components remain the same, what will the effect be?
Broad-sense heritability will decrease.
Broad-sense heritability will increase.
Narrow-sense heritability will increase.
Broad-sense heritability will increase, and narrow-sense heritability will increase.

42. The Limitations of Heritability
24.3 Heritability Is Used to Estimate the Proportion of Variation in a Trait That Is Genetic
The Limitations of Heritability
Heritability does not indicate the degree to which a characteristic is genetically determined.
An individual does not have heritability.
There is no universal heritability for a characteristic
Even when heritability is high, environmental factors may influence a characteristic.
Heritability indicates nothing about the nature of population differences in a characteristic.

43. 24.3 Heritability Is Used to Estimate the Proportion of Variation in a Trait That Is Genetic
Locating Genes That Affect Quantitative Characteristics
Mapping QTLs
Genomewide association studies

44. Figure Mapping quantitative train loci by linkage analysis can help identify genes that help determine differences in quantitative traits.

46. 24.4 Genetically Variable Traits Change in Response to Selection
Natural selection arises through the differential reproduction of individuals with different genotypes.
Artificial selection: selection by promoting the reproduction of organisms with traits perceived as desirable.

47. Figure Artificial selection has produced the tremendous diversity of shape, size, color, and behavior seen today among breeds of domestic dogs. This diagram depicts the evolutionary relationships among wolves and different breeds of dogs from analyses of DNA sequences.
[J. & C. Sohns/AgeFotostock.]

48. 24.4 Genetically Variable Traits Change in Response to Selection
Predicting the Response to Selection
The extent to which a characteristic subject to selection changes in one generation
Factors influencing response to selection
Selection differential
Calculation of response to selection
R=h2 x S
Estimating heritability from response to selection
H2=R/S; realized heritability

49. Figure In a long-term response-to-selection experiment, selection for oil content in corn increased oil content in one line to about 20%, whereas it almost eliminated it in another line.

50. 24.4 Genetically Variable Traits Change in Response to Selection
Limits to Selection Response
Response may level off after many generations
Correlated Responses
Phenotypic correlation
Genetic correlation

51. Figure The response of a population to selection often levels off at some point in time. In a response-to-selection experiment for increased abdominal chaetae bristle number in female fruit flies, the number of bristles increased steadily for about 20 generations and then leveled off.